Stuffed courgettes

• [FEATURE] Finalize implementation of selectChoose and selectSearch that work in all supported
  versions of ember. Added docs for them.
• [FEATURE] Add initial implementation of selectChoose acceptance helper to make
  interaction with the component nicer.
• [FEATURE] Add ARIA roles for basic accesibility. Still more work on this area.
• [FEATURE] Expose test helpers to the consumer app to make integration tests nicer.
• [BUGFIX] Use onmousedown also in clear button (single and multiple modes) to ensure list is not opened.
• [BREAKING] EPS no longer exports the defaultConfig because it's not needed anymore.
  Passing an undefined values for default values does not overrides the default values (null/false do).
  This makes composability easy because creating a wrapper around the component can just forward
  all properties {{#power-select searchEnabled=searchEnabled selectedComponent=selectedComponent}}
  without worrying about if this values is defined or not.
• [BUGFIX] On a select with a selected value, if the selected value is not among the results, the
  first results becomes the highlighted one. Before this fix none was highlighted.
• [BREAKING] The event the triggers the selection of an item is mouseup, not click, meaning that the only
  thing that matters is where the finger is lifted. This is how real selects work in chrome/safari/firefox,
  and so should this. Given that the component opens on mousedown, this allows the user to open and select
  with only one movement.
  Real usage shouldn't break, not acceptance tests, but integration tests using $().click() will. Replace
  this $().mouseup().
• [FEATURE] EPS now accepts a triggerClass which is applied to the trigger.
• [FEATURE] All actions (onchange, onkeydown and onfocus) now receive a richer public API object
  that is identical in shape to the one they received before but also contains highlight(option)
  and search(term) actions
• [BREAKING] Delegate the rendering of the list's topmost element to optionsComponent. This
  allows a better customization of the list. If you use optionsComponent make sure you make it render
  the topmost element, e.g an <ul>.
• [BREAKING] Update to ember-basic-dropdown 0.7.0-beta.1. This means that the component is opened/
  closed using mousedown instead of click. This makes the component feel more snappy. It is unlikeliy
  that this breaks real world usage but might break integration tests of people where people rely
  on $('.ember-power-select-trigger').click().
• [FEATURE] New action: onfocus. Unsurprisingly it is invoked when the component gains focus.
  It receives (dropdown, event) and can be used, by example, to open the component on focus.
• [FEATURE] EPS now accepts a opened boolean property used to open/close the component
  without triggering events on it. Useful to render the component already opened.